The following description sets forth the inventor's knowledge of the related art and problems therein and should not be construed as an admission of knowledge in the prior art.
Conventional adhesives including natural rubber latex and acrylic ester resins are so-called oleaginous adhesives. Because they have strong water repellent properties, oleaginous adhesives neither absorb nor transmit water when applied to an organism for a extended time, particularly during perspiration. Instead, the water is stored between the skin surface and the adhesive tape, causing sweating and rashes. In addition, during profuse perspiration, adhesion sometimes decreases. Further, the adhesives may not adhere to wet surfaces of adherence object. Therefore, it is difficult to apply the tape until the wet surface is wiped or dried.
Also, it is difficult to use oleaginous adhesives as conductive adhesives on an electrode for use on an organism, for example electrocardiogram electrodes or electrodes for performing electric stimulation of low or middle frequency treatment. Instead, hydrogel adhesives commonly are used.
However, conventional hydrogels have several problems. For example, although hydrogels exhibit strong adhesion to strongly hydrophilic materials such as paper, cotton, and the like, hydrogels exhibit detachability rather than adhesion to oleaginous materials such as olefin films and the like. Because sebum, which is oleaginous rather than hydrophilic, usually exists on the surface of human skin, adhesion of hydrogels with highly hydrophilic properties to the skin is limited. In particular, because initial tackiness after application is weak, the applied position of the adhesive may become misaligned or the adhesive may detach immediately after the adhesive tape is applied to the organism. Therefore, additional attachment means are necessary such as temporary fixation with other adhesives. Consequently, hydrogels have been difficult to apply in the medical field.
Furthermore, when electrodes are placed on an organism, they are often placed in pairs with set intervals or distances separating the electrodes. For example, electrodes for measuring body fat percentage that can be applied at certain intervals are known. Specifically, the electrode may have a structure in which a pair of electrode element portions are formed of a conductive material on the surface of a non-conductive substrate sheet. An electrode terminal portion may be formed integrally with the pair of electrode element portions and a conductive adhesive gel layer may be applied on said pair of electrode element portions (see, e.g., Japanese Unexamined Laid-open Patent Publication No. Hei 9-313456).
The electrode for measuring body fat percentage is used by closely contacting the electrode with the skin of a person. The person's body fat percentage is measured by applying a weak current to the body and measuring the electric resistance. For this reason, the electrode for measuring body fat percentage is provided with a structure in which pairs of electrode element portions with intervals of about 10 to about 100 mm are coated with conductive adhesive layers to effect close contact between the electrode element portions and the skin. In order to keep the distance between electrode element portions constant, non-stretchy and comparatively flexible films such as polyethylene terephthalate (PET) preferably are used as the substrate sheet.
Improved adhesiveness of the hydrogels is required in other uses as well. For example, in order to increase initial tackiness, softening a gel has been described. However, when the gel is softened enough to obtain satisfactory initial tackiness, workability of the gel, such as cutting and the like, decreases. This may result in many problems, for example the gel may protrude from the processed product and adhere to wrapping bags and the like; during use, the gel may adhere to clothes or break; furthermore, the gel may protrude from the slit surface during storage; moreover, dust adheres to the gel.
A method of enhancing adhesive properties of the hydrogels other than softening the gel includes the use of non crosslinkable polyvinyl pyrrolidone as a tackifier (see, e.g., Japanese Unexamined Laid-open Patent Publication No. Hei 3-26777 and U.S. Pat. No. 4,860,754). In more detail, the invention relates to a conductive adhesive material comprising low-molecular-weight plasticizers selected from nonvolatile alcohols and polyhydric alcohols; high-molecular-weight water soluble crosslinkable polymers which are soluble in said low-molecular-weight plasticizers; tackifiers comprising non crosslinkable polyvinyl pyrrolidone; and a satisfactory amount of an electrolyte dopant for imparting conductivity to said adhesive material.
However, polyvinyl pyrrolidone is water-soluble and hydrophilic. Because polyvinyl pyrrolidone does not have a satisfactory affinity for sebum and the like, it is difficult to demonstrate strong adhesion after application to skin.
In order to impart initial tackiness, methods are disclosed such as adding hydrophobic polymers to hydrogels, compounding polyhydric alcohols and water in a matrix of hydrophilic polymer. These hydrophobic polymers are adhesives and have good affinity to sebum and the like and are capable of realizing strong adhesion. For example, an adhesive hydrogel comprising glycerin, water, vinyl acetate-dioctyl maleate copolymer, and methyl salicylate in a matrix with synthetic resins such as polyacrylic acid, polyacrylamide, and the like as a skeleton structure is disclosed by Japanese Unexamined Laid-open Patent Publication No. Sho 57-115253. Among these compositions, vinyl acetate-dioctyl maleate copolymer are hydrophobic polymers and adhesives. These hydrophobic polymers are stated to strengthen the adhesion. Vinyl acetate-dioctyl maleate copolymer is sold, for example, under the trade name “Flexbond 150” by Air Products and Chemicals, Inc.
U.S. Pat. No. 4,588,762 discloses conductive adhesives for biomedical electrodes in which a hydrophilic layer comprising water, an electrolyte, and humectants and a viscoelastic polymer layer that substantially is a hydrophobic polymer is placed in dispersal stability. Said humectants comprise 30% to 60% by weight of water, 5% to 35% by weight of latex emulsion (with a polymer solid content of 50%), 10% to 30% by weight of humectants (glycerin and the like), 1% to 10% by weight of electrolyte, and 0.2% to 10% by weight of aqueous polymer, with a tackifier and a film processability improving agent added to enhance adhesion. An exemplary latex emulsion is “Flexbond 150” sold by Air Products and Chemicals, Inc, as mentioned in Japanese Unexamined Laid-open Patent Publication No. Sho 57-115253.
In the above mentioned hydrogels a hydrophobic polymer emulsion dispersed by emulsification is added to a hydrophilic matrix. In order to form a hydrophilic matrix, a previously polymerized synthetic or natural polymer and other compounding components are mixed and crosslinked using, for example, metal or radiation.
Compositions of polymer materials that form interpenetrating polymer networks (IPNs) of polysiloxane (1) and acryl (2) also are known (see, e.g., Japanese Examined Laid-open Patent Publication No. 2641146). In generating the IPNs, a mixture of the monomers and a polymerization initiator or the like is required. The polysiloxane and acryl monomers are poured into a die, thereby reacting them. A network of polysiloxane (1) forms first and then a network of acryl (2) is generated by heating the mixture. It further is stated that the wetting property against water is remarkably low, with respect to the surface characteristics of a product obtained by annealing the aforementioned polymer materials.
The stated reason is that the surface of above mentioned products is composed of a layer of network (1) with a thickness of 5 nm and that even when this surface layer comes off due to wear, the layer recovers to the initial surface state by heating in a short time.
Murayama et al. report that other mixtures of polymers or graft polymer, also in IPN, generate a concentration gradient between a surface and an inner portion (see, e.g., Murayama, S. et al, “Hydrophobic and hydrophilic interpenetrating polymer networks (IPNs) composed of polystyrene and poly(2-hydroxyethyl methacrylate) 2. Gradient composition in the IPNs synthesized by photopolymerization”, Polymer Vol 34 (18), 1993). Therefore, the characteristics stated in patent document 6 (Japanese Examined Laid-open Patent Publication No. 2641146) prove the physiochemical phenomena as well.
Further, regarding the mixture of said hydrophilic polymer and hydrophobic polymer, it is thought that affinity to sebum is further strengthened by bleeding of the hydrophobic polymer layer whose surface free energy is lower. However, not in the case of IPN as stated in the above mentioned Patent Document 6 (Japanese Examined Laid-open Patent Publication No. 2641146) and the like, in the case of a mixture of hydrophilic polymer and hydrophobic polymer, or further when it is the case that the hydrophilic polymer is crosslinked and the hydrophobic polymer is in the state of dispersion by emulsification, the concentration gradient between a surface and an inner portion is generated for a shorter period of time.
Including a hydrophobic adhesives and latex in the hydrophilic polymer matrix or emulsifying and dispersing a hydrophobic adhesives and latex in the hydrogel may help to rectify the hydrogel's low affinity to sebum and weak initial tackiness without damaging other beneficial characteristics of the hydrogel such as mild property and conductivity.
U.S. Pat. No. 6,592,898 discloses a hydrogel comprising a hydrophobic polymer in a hydrophilic polymer matrix wherein the concentration of the hydrophobic polymer at the surface of the gel is greater than the concentration of the hydrophobic polymer inside the gel. Adhesion of the bioadhesive composition is greater at the surface than inside of the gel due to the higher concentration of hydrophobic polymer at the surface of the gel.
As mentioned above, in a gel adhesive composition, improvement in adhesiveness is required both for the use as electrodes and for the use as materials other than electrodes.
For example, in the electrode for measuring body fat percentage, a conductive adhesive gel layer is formed on an electrode element portion. The area of adhesion to the human body is smaller than the whole area of the electrode for measuring body fat percentage. In order to improve adhesion to the skin, it is desired to form an adhesive gel layer on portions of the electrode for measuring body fat percentage other than the electrode element portion, for example, on polyethylene terephthalate.
However, because a conductive gel adhesive layer primarily comprises water and hydrogel that stably retains moisturizer and an electrolyte as required in a hydrophilic resin matrix, it has a weak adhesiveness to the polyethylene terephthalate substrate sheet although it has good adhesiveness to an electrode element portion which is obtained by a paste mainly composed of polyester or polyurethane series resin including carbon. When peeled off from the skin, delamination between the substrate sheet layer and the conductive gel adhesive layers occurs.
In other words, it is difficult to obtain gel adhesive compositions that have strong adhesion both to polyethylene terephthalate and skin.
Even in the case where the electrode is not limited to an electrode for measuring a body fat, by including hydrophobic polymer components in a hydrophilic polymer matrix, improved adhesion is attained. Though inclusion of hydrophobic adhesives and latex in a hydrophilic polymer matrix to obtain hydrogel adhesives with a high affinity to sebum and strong initial tackiness to the skin is known, it is difficult to uniformly disperse and stabilize hydrophobic adhesives in a hydrogel having strongly hydrophilic properties.
As mentioned, there are two primary methods for forming a hydrogel structure that includes hydrophobic adhesives. The first method (A) involves mixing adhesives with a highly viscous solution or paste of a natural or synthetic polymer that was previously polymerized. The dispersed mixture is coated and molded into a sheet form and crosslinked by exposure to radiation and the like to generate a gel. The other method (B) involves emulsifying and dispersing hydrophobic adhesives in a monomer-mixed solution before polymerization. The monomer/hydrophobic adhesive mixture is exposed to light in order to initiate polymerization and cross linking to generate a gel.
In either method, the pre-mixed solution primarily is composed of hydrophilic components and water. Therefore, it is preferable that the hydrophobic adhesives to be added to the pre-mixed solution are added in the form of an emulsion which is emulsified and dispersed in water beforehand. Although method A has the advantage in that the pre-mixed solution has high viscosity and the added emulsion is hard to separate with time, it is difficult to uniformly disperse the emulsion in the pre-mixed solution and the generation of some granular aggregates cannot be avoided. If a stirring method applying a high sheer force is used to destroy the granular aggregates, not only will the emulsion particles be destroyed, but also molecular chains of the polymer which is to be the matrix structure are destroyed. High sheer force stirring thereby risks deteriorating various post-crosslinked gel properties. Therefore, where the post-crosslinked gel properties are to be preserved, some granular aggregates are permitted to exist in the gel, thereby forming a nonuniform gel structure. In method B, while there is advantageous very little risk that granular aggregates will be generated, when the stirring stops, the added emulsion easily separates and the uniformity of the pre-mixed solution is damaged in a short time. In addition, method A does not ensure that the mixture will not separate, though separation occurs more slowly with method A than with method B.
Gels formed in a state where the pre-mixed solution is separated have variable emulsion contents. Since the other composition is also uneven depending on a portion, a gel has very unstable qualities. Further, with the passage of time, the separation state of the pre-mixed solution changes, which increases the variation in the emulsion content of the gel.
Separated pre-mixed solution may sometimes be re-dispersed by re-stirring. In many cases, however, particles of the emulsion become bonded to one another, resulting in increased particle diameters or coagulation, which are irreversible processes. Therefore, countermeasures to prevent the separation of the pre-mixed solution should be taken, for example conducting gelation right after the preparation of the pre-mixed solution or by continuously stirring after the preparation to gelation.
However, in taking these countermeasures, minimum dispersal stability is needed. Even when continuously stirring, this cannot be the preferable method physically and economically, however, because many factors need to be considered, such as containment size, shapes, stirring method, speed and the like. Further, when a gel structure is formed while stirring, large amounts of air bubbles may be mixed into the gel, or, depending on the kinds of monomers used, the amount of dissolved oxygen may change, thereby causing the reactivity and physical properties of the gel to change.
Regarding the B method, although methods such as adding hydrophilic polymer to lower the separation speed or regulating viscosity by adding aqueous plasticizer used as moisturizer may be utilized, it is necessary to be careful when adding a viscosity regulator that emulsion dispersion is inhibited and coagulation is generated. Further, adding salt to the gel to provide conductivity risks salting out and complete separation and coagulation of the gel. This reveals that the process of generating particle mass by taking the A method progresses at an accelerating speed in pre-mixed solution with high fluidity.
In addition, said moisturizer, besides regulating viscosity of the pre-mixed solution, has the important role of maintaining the moisture content of the gel. Therefore, unless a certain amount of moisturizer is compounded, the gel finally obtained has a low moisture content, dries out quickly, has weak adhesion properties, and deteriorating electrical impedance properties.
In other words, because in gel adhesive compositions disclosed in above-mentioned documents that include hydrophobic resin in a hydrophilic polymer matrix hydrophobic resin is easily separated in pre-mixed solution in the production process without continuous stirring, gel adhesive compositions that are strongly adhesive to oleaginous surfaces cannot easily be obtained and continuous manufacturing of the gel adhesive compositions is difficult.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.
The preferred embodiments of the present invention have been developed in view of the above-mentioned and other problems in the related art. The preferred embodiments of the present invention can significantly improve upon existing methods and/or apparatuses.
Among other potential advantages, the object of the present invention is to provide gel adhesive compositions which are excellent in adhesion. In particular, the object of the present invention is to provide gel adhesive compositions having good adhesion to both substrate sheet surfaces which are polyethylene terephthalate surfaces and carbon coat surfaces which are electrode element portions.
In addition, the object of the present invention is to provide gel adhesive compositions and method of manufacturing the same having excellent adhesion to a skin surface of a human body. Further, the object of the present invention is to provide electrodes which include these gel adhesive compositions as conductive gel adhesive layers.